Two types of polyurethane-based membrane are currently used, namely a thermoplastic polyurethane such as Desmopan.RTM. from Bayer or a polyurethane combined with rubber such as Urepan.RTM.. The endurance behavior of these membranes is satisfactory, but they have a drawback as regards the low impermeability to nitrogen of polyurethane. This low impermeability leads to accumulators of limited lifetime. Thus, after three years of use on a vehicle, nearly half of the rating pressure is lost.
It is sought to improve the impermeability to nitrogen of membranes. EP-A-360,648, in particular, discloses a membrane having an impermeability twice as high as the polyurethane-based one. This membrane contains two materials in combination, the first material giving the membrane the required elasticity and being chosen from thermoplastic polyurethanes, polyether block amides and flexible polyesters or a mixture of these, the second material being immersed in the bulk of the first material in order to achieve the impermeability to gases and chosen from an ethylene/vinyl alcohol copolymer, polyamides and polyvinylidene chloride or a mixture of these.
When an ethylene/vinyl alcohol copolymer is used as the second material, it is mixed with a polyether block amide and the first material is a thermoplastic polyurethane. The thickness of the film is, in this case, typically between 10 and 200 micrometers.
Novel hydropneumatic accumulators have appeared recently on the market. They are described in FR-A-2,662,638. They contain two compartments separated by a cold-resistant multilayer membrane forming a gas barrier. The film forming the gas barrier is of the polyvinyl alcohol type placed between two sheets of rubber. The role of the rubber is to ensure the leaktightness of the mounting of the membrane in the accumulator. The polyvinyl alcohol film contains a polyol-type plasticizer, for example glycerol, in a percentage of from 15 to 50% by weight.
These novel membranes are satisfactory, but, owing to the fact that the plasticizer can migrate or evaporate, especially in use at high temperatures, it is necessary to place a skin of vapor-impermeable and plasticizer-impermeable resin, for example a polyethylene skin, between the film of polyvinyl alcohol and the rubber. Furthermore, these membranes require specific storage and handling conditions, owing to the presence of plasticizer.
FR-A-2,685,740 describes flexible membranes for hydropneumatic accumulators, which consist of a layer (1) of ethylene/vinyl alcohol (EVOH) copolymer placed between two layers of polyamide (2), the triple-layer itself being coated on each face with a layer of NBR rubber (3). In order to soften the layers of polyamide (2), it is necessary to add EPDM thereto. The layers (2) do not adhere well to the layer (1). It is also necessary to place an adhesive between the layers (2) and (3).